The present invention relates generally to fuel systems for gas turbine engines. More particularly, the present invention relates to systems for delivering fuel to nozzles within combustors of the gas turbine engines.
Combustors within gas turbine engines are generally of the annular configuration wherein an inner diameter wall circumscribes the engine centerline and an outer diameter wall circumscribes the inner diameter wall to define a combustion chamber therebetween. A ring-like dome typically connects the walls at their upstream end. Fuel nozzles are provided in the dome to inject fuel into a flow of compressed air flowing through the dome. The fuel is injected through small orifices that atomize the fuel to increase combustion efficiency. The nozzles are distributed within the dome evenly around the circumference of the combustor. Recent advancements in combustor design have incorporated the use of primary and secondary fuel nozzles to better control fuel injection during low flow operating states, such as during ignition, ground idle and flight idle. A few primary nozzles that are used during the low-flow conditions are dispersed around the dome and have small injector orifices. A greater number of secondary nozzles having larger orifices are interspersed between the primary nozzles and are brought into use at higher flow conditions, such as during take-off and cruise. The pressure required to properly atomize the fuel in the primary and secondary nozzles can vary widely due to the difference in orifice size.
The primary nozzles open first when fuel flow is initiated, then the secondary fuel nozzles open as fuel flow increases. The valve maintains a minimum backpressure to the primary nozzles to ensure atomization at low flow conditions. The high atomization pressure required by the primary nozzles therefore requires the valve to have a high opening pressure, thereby introducing a point of high pressure drop at the valve during all operating conditions of the engine when the valve is open. It is, however, undesirable to have such a high pressure drop located within the fuel flow. For example, a high pressure drop within the system increases the working pressure and power of the fuel pump, which introduces heat into the fuel system. The heat is an indication of fuel flow inefficiency and, in any event, must be dealt with or dissipated by the engine fuel management system. There is, therefore, a need for controlling flow to primary and secondary nozzles within gas turbine engine combustors without introducing unnecessary high pressure drops within the system.